Microbial abilities to degrade global environmental plastic polymer waste are overstated

Abstract

Internationally, the environmental damage caused by the improper disposal of approximately 100 Mt of plastic waste per annum is of growing concern. Attempts to address this issue have generated many hundreds of scientific studies announcing the discovery of novel plastic-degrading microorganisms and their respective enzymes. On closer inspection, however, evidence remains sparse for the microbial degradation of most of the plastic polymers produced globally. We systematically surveyed the international literature to confirm how many microorganisms proposed to degrade plastics (n = 664) cause substantial (i.e.,  20% mass) losses of virgin polymer, rather than losses of plastic additives, filler, and/or shedding of polymer micro-fragments. Additionally, we noted where degradation was only demonstrated for artificially aged polymer since physicochemical ageing procedures increase the abundance of monomers and oligomers such that they may be degraded by microbial activity. Additionally, artificial ageing may introduce functional groups to the polymer backbone, creating more locations susceptible to microbial degradation than would otherwise occur in the environment. We identified multiple studies demonstrating the effective microbial degradation of heterochain plastic polymers such as polylactic acid, polycaprolactone and polyethylene terephthalate (i.e., polymers containing elements other than carbon in the backbone structure). However, in the literature, we find no evidence for the substantial degradation of unadulterated polyethylene, polypropylene, polystyrene or polyvinyl chloride, homochain polymers which represent the overwhelming majority of global plastics production. Current research demonstrates that the pre-treatment of plastics with elevated temperature or UV-light may speed physicochemical plastic degradation, with valuable applications for downstream microbial processing. However, evidence for the microbial degradation of most plastic polymers in current circulation is lacking. We outline simple criteria that should be met before announcing the microbial degradation of plastic polymers. We hope this may help to address largely unsubstantiated expectations that microorganisms can degrade many plastic polymers in situ.

Into the Plastisphere, Where Only the Generalists Thrive: Early Insights in Plastisphere Microbial Community Succession

“…These conditions are chosen to purposefully alter the physicochemical structure of the polymer. Significant polymer degradation is rarely or never demonstrated under ambient environmental conditions (Lear et al, 2022) and might not be possible under the natural conditions to which our plastics were exposed.…”

“…Further, since the abundance of putative plastic-degraders was equally high in glass-associated biofilms it remains possible that none of the taxa identified have the capability to metabolise PA or LLDPE. Hundreds of publications identify putative PA and LDPE-degrading taxa, yet Lear et al (2022) recently postulated that, in fact, there remains no strong evidence for their microbial degradation. Gangoiti et al (2012); Jeon and Kim (2013) and Montazer et al (2018) reported the mesophilic bacterium Stenotrophomonas maltophilia to be a potential biodegrader of poly(3hydroxydecanoate) (P3HO), PLA and LDPE, respectively, after isolating this bacterium from soil or landfill.…”

Evaluation of the Deterioration of Untreated Commercial Polystyrene by Psychrotrophic Antarctic Bacterium

“…Nonetheless, the substantially analogous weight loss percentage and microbial counts were observed in each of the intervals throughout the experiment. The results may imply the utilisation of the smaller polymer monomers (styrene) or the plastic additivesa series of small molecules that are added to the polymer to aid the manufacturing processes of the final plastic products [38][39][40][41]. The near-constant percentage of weight loss of ≤20%-one of the criteria often considered as a standard for the confirmation of plastic polymer degradation-may also corroborate the potential utilisation of the said additives [41].…”

“…The results may imply the utilisation of the smaller polymer monomers (styrene) or the plastic additivesa series of small molecules that are added to the polymer to aid the manufacturing processes of the final plastic products [38][39][40][41]. The near-constant percentage of weight loss of ≤20%-one of the criteria often considered as a standard for the confirmation of plastic polymer degradation-may also corroborate the potential utilisation of the said additives [41]. Nonetheless, the substantially analogous weight loss percentage and microbial counts were observed in each of the intervals throughout the experiment.…”

Effects and Impacts of Different Oxidative Digestion Treatments on Virgin and Aged Microplastic Particles

“…However, most methodological studies tested treatments on virgin MP rather than aged, neglecting their effect on fragile and damaged particles, more representative of reality [ 28 , 29 , 30 ]. Despite the recent development of biodegradable plastics, less impactful on the environment [ 34 ], many biotic and abiotic factors act on plastics and MPs, leading to changes in polymer properties through different degradation mechanisms [ 35 , 36 , 37 , 38 , 39 ]. Light and temperature, for example, involve free radical formation, chain scission, and subsequent reduction of molecular weight.…”